Traditional cross direction (CD) profile control in the papermaking industry relies on a supervisory level of control based on data acquired by downstream scanning sensors to measure sheet properties such as weight, moisture and caliper. The measured sheet properties are then used to manipulate upstream actuators distributed across the sheet to correct deviation from a desired target profile for a particular measured property. The scanning sensors are usually located at the dry end of the paper machine and it can take several minutes for the scanners to detect changes in the sheet made by the actuators located at the wet end of the machine due to the delay in the paper sheet travelling from the actuators to the sensors.
In paper making machinery, stock which is essentially a fibre suspension is fed from the head box through a gap or elongate orifice whose configuration can be controlled by actuators onto the wire section. The gap is a relatively narrow opening that extends across the width of the machine. The major components that make up the orifice comprise a bottom section, referred to as the apron, and a top section, referred to as the slice lip. Weight profile control in such an arrangement is generally achieved by adjusting the position of the slice lip across the machine with motorized actuators to vary the shape of the gap or orifice. The actuators usually have some form of feedback of the position of the actuators relative to some "zero" position. Applicant is aware of a number of patents directed to actuators and control systems for manipulating the slice lip as follows:
U.S. Pat. No. 4,517,055
U.S. Pat. No. 4,770,744
U.S. Pat. No. 4,883,941
U.S. Pat. No. 5,096,542
These actuators and control systems are conventional in that they rely on downstream scanners providing control signals to independently adjust the position of each actuator in order to adjust the measured profile toward a desired target.
In the past few years, new weight profile control systems have been developed which locally dilute the stock in the headbox using dilution actuators which are controlled by the downstream scanning actuators. In both conventional and new systems, the desire is a uniform profile across the sheet when measured by the scanning system. Since the fibre distribution in the stock depends on the stock consistency across the head box and on the slice-to-apron opening across the machine, it is necessary to ensure a uniform opening from the slice lip to the apron. Most dilution profile control systems are purchased to provide narrower CD profile control than conventional slice lip control systems and to provide uniform fibre orientation across the sheet since the slice-to-apron opening (gap) can be held constant in theory. In practice, it has been found that it is very difficult to maintain a uniform slice-to-apron opening due to thermal, mechanical and other instabilities, and it continues to be necessary to adjust the gap for fiber orientation and, in some cases, for weight profile control.